Conventionally, when installing the electric power steering apparatus on the vehicle, the arrangement of each component thereof is mirror-reversed with a longitudinal centerline of the vehicle as an axis between when installed on a right-hand-drive vehicle and on a left-hand-drive vehicle. Arranging in such manner is hereinafter referred to as a mirror-reversed arrangement.
Then, due to the mirror-reversed arrangement, the correspondence between direction of rotation of a steering handle and direction of rotation of the electric motor for generating assist force is reversed. Therefore, it is necessary to change the content (software) of a control unit (e.g., ECU) for controlling the power supply to the electric motor.
This is described in more detail with reference to FIGS. 1 and 2. FIG. 1 shows the electric power steering apparatus carried on the right-hand-drive vehicle. Turning operation of a steering handle 101 by a driver is transferred to a rack shaft 104 through a steering axis 102 and a pinion part 103.
The rack shaft 104 is coaxially provided with an electric motor 105 for assisting lateral (axial) movement of the rack shaft 104 and with a rotation angle sensor 106 for detecting the rotation angle of the electric motor 105. The electric motor 105 shall be provided with a right-handed reduction gear (not shown) in this context.
A steering torque sensor 107 is provided on the middle of the steering axis 102. The steering torque detected by the steering torque sensor 107 is transferred to an ECU 108 for controlling the power supply to the electric motor 105. The ECU 108 is configured to control the rotation of the electric motor 105 in accordance with the direction and amount of steering torque (i.e., with the direction and amount of steering operation) to generate the assist force depending on the steering torque.
Now, it is assumed for the steering handle 101 that turning to the left (in the counterclockwise direction) from the driver's standpoint is defined as turning in a forward (+) direction and that turning to the right (in the clockwise direction) from the driver's standpoint is defined as turning in a reverse (−) direction. Further, it is assumed for the direction of rotation of the motor that the direction of rotation in which the motor generates an assist force to push the rack shaft 104 to the left is defined as a forward (+) direction and that the direction of rotation in which the motor generates an assist force to push the rack shaft 104 to the right is defined as a reverse (−) direction.
Thus, in an example shown in FIG. 1, the ECU 108 for controlling the power supply to the electric motor 105 is programmed to rotate the electric motor 105 in the forward (+) direction when the direction of steering torque is forward (+) and to rotate the electric motor 105 in the reverse (−) direction when the direction of steering torque is reverse (−).
FIG. 2 shows the above-described electric power steering apparatus installed on a left-hand-drive vehicle with mirror-reversed arrangement. The same components have a consistent reference number.
As shown, the correspondence of the direction of steering to the direction of rotation of the motor is reversed due to the mirror-reversed arrangement. Therefore, when the ECU 108 controls the power supply to the electric motor 105 in a similar manner as in when installed on a right-hand-drive vehicle (i.e., the ECU 108 is carrying the same software for control as in when installed on the right-hand-drive vehicle), the rack shaft 104 is assisted to move to the right when steering to the left and assisted to move to the left when steering to the right. Briefly, the rack shaft 104 is assisted in incorrect directions.
Consequently, the program installed on ECU 108 has to be changed when the above electric power steering apparatus is installed on the left-hand-drive vehicle, such that the ECU 108 is programmed to rotate the electric motor 105 in the reverse (−) direction when the steering torque is in the forward (+) direction and to rotate the electric motor 105 in the forward (+) direction when the steering torque is in the reverse (−) direction.
Practically, various methods can be employed for this, including (1) two types of ECUs for use in the right-hand-drive vehicle and in the left-hand-drive vehicle are prepared and installed appropriately in a manufacturing process, or (2) two types of software for use in the right-hand-drive vehicle and in the left-hand-drive vehicle are installed on the ECU and an appropriate one is selected to use by obtaining information on the steering handle from the vehicle after the installation.
However, with a conventional method like the above (1), incorrect installation may happen since the ECUs for use in the right-hand-drive vehicle and in the left-hand-drive vehicle generally have the identical shape in appearance. Once an incorrect ECU is installed, the steering handle oscillates and it is impossible to steer properly. More particularly, it is difficult for the driver even to steer such vehicle because the rack shaft is assisted in the opposite directions. Therefore, it is difficult for the driver to drive such vehicle even from the exit of the manufacturing line to a place for repair and part replacement.
Also, with the above (2) method, it is impossible to control the vehicle when an EEPROM for loading the information about the steering handle from the vehicle fails.